One widespread transponder application are contactless portable data carriers which cooperate with a reading device that contains a coil, with the help of which the reading device performs the power supply for the transponder brought into the response range and the readout. As to be able to identify the bringing near of a transponder the reading device cyclically at short time intervals produces a magnetic field which is suitable for supplying a transponder with power that, optionally, is brought within the addressing range. At the same time the reading device usually sends out one interrogation signal at a time with which a transponder is addressed. The regular production of magnetic field and interrogation signal causes a comparatively high energy consumption which renders the concept unsuitable for applications in which a sufficiently large power supply cannot be made available.
Furthermore, in DE 100 06 747 A1 can be seen a generic device which especially deals with the problem of power consumption. It is proposed to equip a portable transponder element with a permanent magnet which when approached to a reading apparatus actuates a switch disposed therein that is controlled by a magnet. The proposed device minimizes the power consumption of the reading apparatus since the latter can remain completely switched off during the absence of a transponder. The installation of a permanent magnet requires constructional measures regarding the transponder elements to be equipped with such a magnet, and such measures cannot always be taken easily. The mechanic integration is problem enough as to render the solution not suitable for, for example, contactless chip cards. Furthermore, the magnetic field produced by a permanent magnet often is undesirable with regard to the practical usability of the transponders equipped with such permanent magnets. This applies for example to portable data carriers in chip card format where information is stored on a magnetic stripe. The handling of such transponders is also restricted in so far as they have to be kept away from other circuits sensitive to magnetic fields.
The book “RFID-Handbuch” written by K. Finkenzeller, Carl Hanser Verlag, 2nd edition, 2000, describes in detail the basic principles of the transponder technology and shows examples of transponder applications. In particular chapter 4 of this book gives basic information and additional explanations to the invention described hereinafter. Explicit reference is made to these passages in particular and to the book as a whole, they shall be part of this application.
From DE 196 02 316 C1 there is known a device for the transmission of data and supply power from/to a transponder which can be used for example in a theft protection system of a motor vehicle. The device has a fixed transceiver as well as a portable transponder which when approached to the transceiver cooperates with it. To achieve a power or data transmission as effective as possible it is proposed after the manufacturing of the transceiver to adjust the resonant frequency in the transmitting antenna circuit and/or the quantity of the exciting current flowing in the transmitting antenna circuit in such a manner that the result is a maximum power transmission to the transponder. The found adjustment is fixed by circuit-technical means.
From DE 199 23 367 A1 a device for the non-contacting position recording of an object is known which has a transmitting antenna, a receiving antenna and an evaluation circuit. Here transmitting antenna and receiving antenna influence each other. With the presence of an object their coupling changes. The change is recorded and evaluated.